The present invention relates to a self-supporting sandwich structure element comprising at least one solid polyurethane layer and at least one polyurethane foamed layer.
It has already been proposed, according to WO 94/14587, that sandwich structure elements comprising two-component reaction plastics be produced by the spray application in layers of corresponding reactive mixtures on to a substrate. Polyurea and mixtures of polyurea and polyesters are described as the reaction plastics. Structural elements such as these may consist of two solid reaction plastic layers and of a reaction plastic foamed layer situated therebetween. The advantages of producing sandwich structure elements by spray application in layers of two-component reactive systems result from the rapid reaction time of these systems, their processability at ambient temperature and the insignificant demands made on the mold or on the substrate. In addition, they can be produced from a homogeneous material, wherein the reactive mixture merely has to be modified by the addition of a foaming agent in order to produce the intermediate foamed layer.
It has also already been proposed, according to European patent 589,343, that sanitaryware articles, the visual and usable surfaces of which consist of polymethyl methacrylate ("PMMA"), be provided with a supporting polyurethane sandwich structure, application of the layers being effected directly on to the premolded polyurethane substrate, in layers, by means of a two-component mixing nozzle.
One problem which is addressed in the '343 reference is the adhesion of the polyurethane sandwich structure to the PMMA layer. It is proposed according to the '343 reference that the PMMA substrate be treated with a solution of an uncrosslinked, elastic polymer resin having a strong crystallization tendency. Other proposals aimed at solving the problem of adhesion between polyurethane and PMMA are referred to as prior art in the '343 reference.
A further problem associated with rapidly reacting polyurethane systems is the heat evolved during the polyisocyanate addition reaction, which cannot be dissipated rapidly enough due to the rate of the reaction and due to the thermal insulation properties of the foamed layers which are built up with the sandwich. Temperatures of 150.degree. C. or more can easily be reached, depending on the layer structure of the sandwich. This temperature increase during the curing of the sandwich structure is particularly disadvantageous when the sandwich structure is applied to a thermoplastic material; the temperature during the coating of PMMA must not exceed 80.degree. C., for example.
In principle it should be possible to develop mixtures which react to form polyurethane which cure more slowly due to a reduced content of catalyst. In systems such as these, however, the commencement of the reaction would also be slowed down, so that the initial flow behavior would be altered, with the effect that run-off would occur when applying the mixtures to vertical surfaces or surfaces sloping in relation to the horizontal.
Another problem, particularly during the manufacture of sandwich structure elements of large surface area, is that of distortion defects, which are due to shrinkage effects during the curing of the mixture which reacts to form polyurethane. When structural elements are produced in layers, shrinkage within the layers occurs at different times.